Fluid metering systems requiring high accuracy flow, such as fuel metering systems for gas turbine engines, often use a servo-controlled throttling valve to maintain a constant pressure delta (head pressure) across a metering orifice. In such systems, a supply of burn fuel is directed against a first side of a throttling valve, and the position of a valve body in the throttling valve controls the amount of fuel that leaves the valve outlet.
A servo may be used to create a control pressure (“PX pressure”) for positioning the valve body to vary the flow rate through the valve. In high pressure systems, the fluid conveying the control pressure against the valve body may leak between the valve body and the valve sleeve and into the valve outlet. This “matched clearance servo leakage,” adds to the amount of servo flow required for the valve to hold position, as well as, adding to the measured volume of burn fuel that passes through the valve outlet. Because this leakage is somewhat unpredictable it varies with temperature and operating conditions, for example, it is often a large contributor to metering system inaccuracy.
In order to control matched clearance leakage, one known throttling valve includes a dual outer diameter valve body and a dual inner diameter valve sleeve. FIG. 6 illustrates a conventional dual-diameter throttle valve assembly 210 comprising a valve housing 212 and a valve sleeve 214 in which a valve body 216 is slidably mounted. Valve sleeve 214 includes a first inner diameter 218 and a second inner diameter 220 while valve body 216 includes a first outer diameter 222 corresponding to valve sleeve first inner diameter 218 and a second outer diameter 224 corresponding to valve sleeve second inner diameter 220. Seals 226 and 270 control leakage from the supply flow path 286 and a leakage control port 228 during shutoff. The leakage control port 228 ports supply pressure to a center area between the valve body and the valve sleeve to reduce the pressure differential between the control pressure flow path 290 and the outlet flow path 288 to reduce servo leakage.
System requirements typically call for drip-tight shutoff leakage on throttle valves. Not only is the dual-diameter matched valve assembly expensive to manufacture, but it is extremely difficult to achieve the low leakage or drip-tight shutoff sealing required in some applications with this valve. In the dual-diameter configuration, it is also extremely difficult to install a durable drip-tight seal in the upper seal diameter, because the seal 226 must be stretched when it is installed and re-sized after installation. Moreover, burn fuel from the supply flow path 286 which supplies flow to leakage control port 228, tends to contain more impurities than fuel from the control supply flow path 290 which has generally been more thoroughly filtered. Seal 226 is exposed to the burn fuel, and these impurities may tend to collect around seal 226, thus degrading performance of the seal and interfering with a drip-tight shutoff. It is therefore desirable to provide a valve assembly that does not suffer from the above-described shortcomings.